1. Field of the Invention
The present invention relates generally to the field of power supplies and more specifically to multi-voltage power supplies suitable for use in a universal airplane ground support equipment cart.
2. Description of the Related Art
When an airplane is on the ground with its engines shut down, the airplane is typically unable to provide power for its electrical systems and chilled air for its air conditioning systems; and some airplanes are also unable to provide liquid coolant for some critical electronic (or “avionic”) components. It is customary to connect such a grounded airplane to an airplane ground support equipment system. Such a system may have its components mounted upon a mobile equipment cart that is called an airplane ground support equipment cart and that may be parked, placed, or mounted conveniently close to an airplane requiring ground support. Such a cart typically contains an air conditioner that can provide conditioned and cooled air to an airplane plus an electrical power converter that can transform power drawn from the local power grid into power of the proper voltage (AC or DC) and frequency required by the airplane. Such an airplane ground support equipment cart may also contain a diesel engine connected to an electrical generator that enables the cart to provide both air conditioning and also electrical power for an airplane without any connection to the local power grid. And if an airplane requires a source of cooled liquid for its electronics, some carts may also include a source of liquid coolant, typically a polyalphaolefin, or PAO, heat transport fluid or liquid coolant.
In the past, particularly with regard to military airplanes, such ground support equipment carts have been custom designed to meet the specialized needs of a single particular type or class of airplane. Hence, a cart designed to support the specific requirements and needs of a first type or class of airplane cannot be used to support the differing specific requirements and needs of other types or classes of airplanes. Different airplanes typically may require different pressures and volumes of cooled air, different amounts of electrical power, different electrical voltage levels, and different electrical frequencies (or direct current). And different airplanes typically may require differing pressures and volumes of cooled liquid for use in cooling onboard electronics. Accordingly, every airport must be supplied with as many different types of ground support equipment carts as there are different types or classes of airplanes that may land and take off at each airport or military base. Problems arise when more airplanes of a particular type arrive at a specific location than there are ground support equipment carts suitably designed to service the needs of that particular type or class of airplane.
To be more specific, some airplanes require their ground support equipment to provide considerably more airflow at higher pressures than do other airplanes having smaller interiors. Some airplanes require their electrical power to be adjusted to 115 volts of alternating current (A.C.) which alternates, or flows back and forth, 400 times each second (115 volts, 400 Hz A.C.). Other airplanes require 270 volts direct current (270 volts, D.C.) that does not flow back and forth. Yet other airplanes require a source of 28 volts of direct current (28 volts, D.C.). And airplanes also differ in the amount of electrical power that they draw.
As an example of an airplane cart arrangement that provides air conditioning and electrical power conversion for an airplane, PCT patent application No. PCT/US2006/043312 (Intl. Pub. No. WO 2007/061622 A1 published on May 31, 2007) discloses an airplane ground support cart that has a modular design of its electrical conversion components. This cart provides air conditioning and electrical power conversion as well as optional electrical power generation services to airplanes. FIG. 5 reveals that the cart disclosed in this patent application may receive interchangeable, modular power conversion modules. Thus, a module 72, which generates 3-phase 115 volt 400 Hz A.C. power, may be removed and replaced with a module 78, which generates 270 volt D.C. power. FIG. 6 illustrates that this cart may also accept a module 92, which generates 28 volt D.C. electrical power.
FIG. 2 of the above PCT patent application illustrates a typical arrangement of the mechanical components of a dual air conditioning system within an airplane ground support equipment cart 14. The air conditioner's mechanical components are spread all across the entire length of the cart 14. Two sets of condenser coils 34 are positioned at one end of the cart 14; and the thickness of the coils 34 and their housing, together with the thickness of the associated cooling fans, occupies roughly one-fifth of the cart's overall length. A filter and upstream evaporation coil 30 and a downstream evaporation coil 40 and outlet connection 42 (to which can be attached a duct leading to an airplane) are positioned at the other extreme end of the cart 14, occupying somewhat less than one-fifth of the cart's overall length. A blower fan 32, a discharge plenum 38, and two compressors 36 are shown positioned in the central portions of the cart 14. These mechanical components of the air conditioning system are not confined within a rectangular module within a portion of the volume of the cart 14—these components are spread all across the cart 14 and thus cannot be conveniently removed from the cart for servicing or for use away from the cart 14. Other cart components, such as a diesel engine 54 and generator 56 (shown in FIG. 4 of the PCT application) and an electrical power converter unit 72 (shown in FIG. 5 of the PCT application) are squeezed in among the air conditioning components wherever there is room. This intermixing of non-air-conditioning components with the air-conditioning components greatly complicates servicing of all the components, since they are all crowded into the same cramped space. A service man working on the air conditioner compressors or blowers may find the diesel engine 54 and the generator 56 are in the way of these components, for example.
The air conditioning systems of such a conventional ground support equipment system is also designed to provide a particular volume of cooled air at a particular temperature and pressure to a particular type or class of airplane. If such a system has its cool air ducted into some other type or class of airplane, too much or too little air will flow from the air conditioner system, and this will throw off the balance of the air conditioning system, causing the air to be cooled too little or too much and possibly causing icing of the internal evaporators or damage to the airplane. And the temperature and pressure provided may not be proper for some other type or class of airplane. Likewise, the electrical systems may not be able to supply the needs of differing types or classes of airplanes, and the PAO liquid cooling system may not be properly balanced when used to cool the avionics of differing types or classes of airplanes.